Arbor Day is usually celebrated by planting new trees, so releasing FarmVille Saplings is quite appropriate!

In the United States, Arbor Day is nationally reserved as the last Friday of April.

"Arbor Day was founded in 1872 by J. Sterling Morton in Nebraska City, Nebraska. By the 1920s, each state in the United States had passed public laws that stipulated a certain day to be Arbor Day or Arbor and Bird Day observance. The national holiday is celebrated every year on the last Friday in April; in Nebraska, it is a civic holiday. Each state celebrates its own state holiday. The customary observance is to plant a tree. On the first Arbor Day, April 10, 1872, an estimated one million trees were planted."